In recent years, from the viewpoint of energy conservation, light-emitting diodes (hereinafter, referred to as “LEDs”) have been used as light sources in place of fluorescent light and halogen lamps. Such a light emitting device which uses an LED as a light source may, in some cases, be disposed obliquely with respect to a surface to be irradiated (hereinafter, also referred to as an “irradiation surface”) instead of being disposed directly above the irradiation surface according to applications of the light emitting device.
For example, when a plant is cultivated using a shelf provided indoors, a light emitting device which is disposed obliquely with respect to a plate of the shelf, illuminates the plant disposed on the plate of the shelf. When the light emitting device is disposed obliquely with respect to a planar irradiation surface in this manner and an LED is used as is as the light emitting device, most of light emitted from the LED is undesired light which is not directed to the irradiation surface, and it is impossible to efficiently illuminate the irradiation surface. Further, the undesired light enters the eyes of workers, which makes it difficult for the workers to work. As means for solving the above problem, a lens which controls light distribution of the light emitted from the LED may be used in combination with the LED (see PTL 1, for example).
PTL 1 discloses an illumination apparatus which has a planer irradiation surface, a plurality of LEDs disposed on a straight line which is parallel to the irradiation surface, and a compound lens which covers the plurality of LEDs. In the illumination apparatus of PTL 1, the plurality of LEDs and the compound lens are disposed so that both light-emitting surfaces of the LEDs and an emission surface of the compound lens are perpendicular to the irradiation surface. The compound lens has the same cross-sectional shape at any point of the compound lens in a direction orthogonal to the direction of the disposed LEDs. Further, a shape of an incidence region (region facing the LEDs) of the compound lens is asymmetric with respect to a plane which includes optical axes of the LEDs and is parallel to the direction of the disposed LEDs. In this way, because in the illumination apparatus disclosed in PTL 1, the compound lens has a shape which is asymmetric between a side of the irradiation surface and an opposite side of the irradiation surface, even if the illumination apparatus is not disposed directly over the irradiation surface, the illumination apparatus can illuminate the irradiation surface uniformly to some extent.